10 States Standards Minimum Slopes

[Kozzybear]

Hi all,

I was thinking about how the minimum slopes came to be in the 10 states standards manual for storm and sanitary design criteria. Does anyone know how the slopes that are used came to be? How do we know an 8-inch sanitary line needs a minimum slope of 0.4% and not 0.35%. Just a thought I had that maybe someone with a little more knowledge on the criteria could shed some light on. I'm kind of hoping someone will point me in the direction of published lab results, but if not that's alright too.

Thanks!

 

[Pinwards]

I believe the 0.4% for 8" sewer is to achieve 2.5 fps velocity when flowing full, which is general quoted as the velocity needed to be "self cleaning" to carry sediment downstream, rather than settling out and potentially plugging the pipe over the long term.

#

 

[rconner]

While I guess I can see the temptation based on some contemporary promotions, I think one should be quite careful when contemplating designing sewers at flatter slopes (than e.g. per Ten States standards at

http://10statesstandards.com/wastewaterstandards.pdf)

. That standard and those minimum requirements appear to be not just based on "lab" tests, but instead the experience of utilities and regulators reflected in multiple editions over at least 55 years comprising and/or influencing the Board in the Ten States area. The slopes you talk about are based on a Manning's "n" of 0.13, and e.g. in the common 8" size achieving a 2 fps velocity that it is hoped will move most normal solids along. Also, while later hinting that a lesser slope might be tolerable in at least some cases,the minimum slope table is preceded with the statement, "However, slopes greater than these may be desirable for construction, to control sewer gases or to maintain self-cleansing velocities at all rates of flow within the design limits", and the table is followed up with the further sort of proviso and perhaps caveat, "Slopes that are slightly less than the recommended minimum slopes may be permitted. Such decreased slopes may be considered where the depth of flow will be 0.3 of the diameter or greater for the design average flow. The operating authority of a sewer system considering decreased slopes shall furnish the appropriate reviewing authority written assurance that any additional sewer maintenance required by reduced slopes will be provided."

You may be interested that quite similar requirements and provisions have also been included in ASCE's "Gravity Sanitary Sewer Design and Construction" (WPCF FD-5) Manuals and reports on Engineering Practice, used in much larger areas and jurisdictions also for many years. The latter reference I know explained, "Generally, Manning's "n" for a given sewer, after some time in service, will approach a constant which is not a function of the pipe material but represents the grit accumulation and slime build-up on the walls. This n will be on the order of 0.013. A coefficient which will yield higher friction losses should be selected for sewers where disturbing influences are known or aniticpated. Because of the empirical nature of each formula, conservative design is prudent." [I suspect there is some testing revealing the basic flow, so to speak, of this passage]

About the only other thing I will say is that I have attended a few Engineering presentations (since first reading these manuals many years ago) indicating that as sewers get much larger, even much higher flow velocities in the realm e.g. of at least 3-4 fps or more etc., may be helpful for large sewers it appears to develop sufficient shear to adequately move most sediment including larger grit and even some rocks etc. that somehow seems to make its way into these larger lines.

 

[bimr]

Agree that the slopes are specified to obtain the minimum velocity at full flow.

The problem occurs with new installations where the flow is less than full flow and the sewers are at minimum slope. In this scenario, additional sewer cleaning will be necessary because the sewer velocities are low.

 

[racookpe1978]

Yes, but now that San Francisco and LA have been requiring low-flow and ultra-low-flow toilets for several years, they are finding that even "acceptable" slow sewers are backing up and plugging up with "normal wastes" when enough people and houses/apartments in any single small area buy the new/mandated toilets ... Many blocks in SFO are now smelling, backing up sewage into the basement and lower flows, and overflowing into streets.

Now, CA is not the rest of the country, but the national specifications for toilets, water, gasoline, electric safety, cancer chemicals, and food labelling and seemingly everything else seems to creep east from the CA land.

 

[coloeng]

Just wait and see what low flow toilets do to the plumbing in houses when states start allowing the use of grey water recycling. I know some legislators in Colorado have been pushing for that.

 

[dicksewerrat]

Just remember that some of us get paid well to fix the problems. The bigger the issue the larger the paycheck. another thing I learned a long time ago, the building is not much good if the toilet doesn't flush.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[civilman72]

Dick: Just curious, how do you fix such a problem (i.e. a sewer main at minimum slope clogging because of abundance of low-flow toilets)?

 

[bimr]

This document from 2011 indicates that the concern for sewer issues is somewhat exaggerated.

“With regard to municipal sewer lines, the transport of waste has not proven to be an issue of
concern in those areas with a concentration of high‐efficiency toilets. Supplementary
wastewater flows from other end‐uses are always sufficient to move solids through the system.
Furthermore, some wastewater utilities are co‐funding and sponsoring the toilet replacement
programs and other water efficiency initiatives of the water utilities for the very purpose of
reducing sewer flows to their treatment plants.”

http://www.allianceforwaterefficien...l/toilets/AWE-Drainline-Article-July-2011.pdf

 

[racookpe1978]

But is not the "Alliance for Water Efficiency" an advocacy/lobbyist/advertising/users group intended and funded specifically for such press releases?

Are they actually reliable, or only reliable inside Sacramento and Washington DC's beltway when asking about more money and rules benefiting their clients? 8<)

 

[bimr]

You may or may not be aware that many plumbing requirements are not scientifically established.

he Alliance for Water Efficiency is a stakeholder-based 501(c)(3) non-profit organization dedicated to the efficient and sustainable use of water. Headquartered in Chicago, the Alliance serves as a North American advocate for water efficient products and programs, and provides information and assistance on water conservation efforts.

This work is being led by the Plumbing Efficiency Research Coalition (PERC), a group of U.S.‐based organizations that support water efficiency and sustainable plumbing, in collaboration with the Australia‐based Australasian Scientific Review of Reduction of Flows on Plumbing and Drainage Systems (ASFlow) Committee. PERC members include the Alliance for Water Efficiency (AWE), International Association of Plumbing & Mechanical Officials (IAPMO), International Code Council (ICC), Plumbing‐Heating‐Cooling Contractors National Association (PHCC), Plumbing Manufacturers International (PMI), and the American Society of Plumbing Engineers (ASPE).

Many advocates there.

 

[cvg]

many advocates for reducing water usage, but not one of those groups actually owns and maintains a sewer system

 

[bimr]

Been waiting a long time for a client to say there is a problem with low flow. However, the clients have a never ending problem with too much flow.

 

[rconner]

(and costs to process/treat it, or pay fines etc. for overflows)

 

[dicksewerrat]

If it is a building service line, put a couple of 'water hog' washers in the laundry room.
If it is mainline, reconnect 1-2 Catch basins to the line. If it is in an easement look for a water source. Last resort, shrink the pipe size. Clean on an accelerated schedule. Inspect about once a month to see the buildup. That means "don't jet" the line before you send the camera in.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com